Now why is this? The simple answer is: WID basically uses an older version of SQL than SQL server itself. You can restore a database from a lower version of SQL to a higher version of SQL, but you CANNOT restore a database from a higher version of SQL to a lower version of SQL!

We want to move from ADFS 3.0 SQL to WID, so we have to setup a new farm in parallel with same name and service account. No problem up to there. But how can we export and import all the configuration (relying party trusts, claim rules and stuff). I thought it would be easiest to use the export-federationconfiguration.ps1 on the existing ADFS 3.0 server but it throws an error: Unable to find type [Microsoft.IdentityServer.PowerShell.Resources.RelyingPartyTrust]. Make sure that the assembly that contains this type is loaded.

Is this a supported method to export from ADFS 3.0 and import to a new ADFS 3.0 farm with this cmdlet?

The export script only works in ADFS v2.x as it is meant to migrate from ADFS v2.x to ADFS v3. The reason the export script does not work in ADFS v3 has to do with a reference to an assembly that does not exist
However, somebody fixed the script so that it also works in ADFS v3